Method of making superalloys

ABSTRACT

Nickel-base superalloys are made substantially non-wetting to refractory crucibles by introducing into an essentially sulfurfree and oxygen-free melt of such superalloy enough magnesium to incorporate from 0.001 to 0.02 percent of elemental magnesium by weight in vacuum-cast cooled and solidified ingots or other shapes cast from such melt, and casting and solidifying the magnesium-containing melt in an ingot or other mold.

United States Patent [191 Hulit et a1.

['4 Nov. 26, 1974 METHOD OF MAKING SUPERALLOYS Inventors: Gerald W.Hulit, Ridgewood; John C. Subelka, Dover, both of NJ.

Assignee: Howmet Corporation, Dover, N .1.

Filed: Mar. 6, 1973 Appl. No.: 338,435

U.S. Cl 75/171, 75/135, 164/57,

164/61 Int. Cl C226 19/00 Field of Search 75/135, 171; 164/57, 61

References Cited UNITED STATES PATENTS 6/1958 Kerstetter 75/171 3/l96lEash 8/1965 Scheil et a1. ..T 75/171 3,512,963 5/1970 Schramm et a175/171 3,575,734 4/1971 Muzyka et al. 3,619,183 11/1971 Olson et al..75/171 Primary ExaminerL. Dewayne Rutledge Assistant Examiner-E. L.Weise Attorney, Agent, or FirmPennie & Edmonds [57] ABSTRACT 6 Claims,No Drawings METHOD OF MAKING SUPERALLOYS BACKGROUND OF THE INVENTIONNickel-base superalloys are extensively used in the manufacture ofarticles such as turbine blades which are required in use to resist hightemperatures. Such articles normally are produced by precision castingmethods, which involve melting ingots of the superalloy in a refractorycrucible and casting the resulting melt in a suitable mold.

Quite frequently the superalloy wets the refractory crucible in which itis melted. When this occurs a thin layer of the melt adheres to thesurface of the crucible when molten metal is poured from it, and thenfreezes as a skull on the interior of the crucible surface. In so doingit contracts and pulls away from the crucible, and thereby'tears smallbits of refractory from the crucible wall. When the next charge ofsuperalloy is introduced into the crucible and melted, these bits ofcrucible refractory contaminate the melt and are likely to be in- Vtroduced into the casting made from the melt. As successive charges ofsuperalloy are melted and cast, the amount of refractory particles tornfrom the crucible surface increases, and increased numbers of castingsare made defective by refractory inclusions. Since the metal is meltedand cast in vacuum, and since it is the usual practice to melt in eachcharge only about the amount of meta] required for a single lot ofcastings, it is not practical to remove the skull and loose refractoryparticles from the crucible between successive charges of metal to bemelted.

Some superalloys have a greater tendency than others to wet refractorycrucibles, but in general such tendency is increased in ingots madewholly or partly from foundry revert (i.e., risers, gates, scrapcastings and the like). This tendency is so marked that foundries willoften specify ingots made only of virgin metal (that is, ingots producedwithout the use of revert metal) even though the economic penalty ofsuch specification is severe. No reliable procedure has been proposedheretofore for reducing or minimizing the tendency of nickel basesuperalloys to wet refractory crucibles in which they are melted.

The invention provides a method for overcoming the tendency ofnickel-base superalloys to wet refractory melting crucibles by treatinginto such alloys in the molten state with a small amount of magnesium.Magnesium has been added to certain refractory nickel base alloysheretofore, for the purpose of deoxidizing or desulfurizing them orincreasing their ductility. For example, the Randolf U.S. Pat. No.2,001,888 describes introducing a trace" of magnesium into anickel-magnanese-chromium alloy as a deoxidizer of the alloy and toincrease its ductility. Each U.S. Pat. No. 2,974,038 describes agall-resistant nickel base alloy which may contain 0.3 to 0.l percentmagnesium as a residual deoxidizer. Eiselstein et al. U.S. Pat. No.3,160,500 describes a nickel-base chromemolybdenum-tungsten-columbiumalloy to which 0.02 to 0.05 percent magnesium is added to improve hotmalleability of the metal. There are many other patents and publicationsmentioning the deoxidizing and duetility or malaleability-enhancingproperties of magnesium in nickel base alloys, but we are not aware ofany prior recognition that magnesium treatment of an already deoxidizedand desulfurized nickel-base super STATEMENT OF THE INVENTION Theinvention provides an improved method of making nickel-base superalloys,by which ingots of such alloy having relatively little tendency to wetrefractory melting crucibles may be made. The invention is directedparticularly to the vacuum casting of substantially oxygen-free andsulfur-free nickel-base superalloys consisting essentially (by weight ofthe alloy) of 7 to 20 percent chromium, 0.5 to 7 percent aluminum, 0.5to 6 percent titanium, 0.02 to 0.25 percent carbon, up to 0.2 percentzirconium, up to 0.02 percent boron up to 3 percent hafnium, and atleast 10 percent of an amount of a matrix-strengthening element selectedfrom the group consisting of 0 to 20 percent cobalt, 0 to 10 percentmolybdenum, 0 to 13 percent tungsten, 0 to 6 percent tantalum, 0 to 5percent columbium, and 0 to 1.5 percent vanadium, and the balancenickel. The invention provides the method of minimizing the tendency ofsuch alloys to wet and upon solidification to adhere to a refractoryvesse in which it is contained under vacuum in the molten condition,which comprises introducing into a substantially oxygen-free andsulfur-free melt of such alloy an amount of magnesium sufficient toincorporate in cooled and soldified shapes cast therefrom a residualretained amount of elemental magnesium in the range from 0.001 to 0.02percent by weight, and casting and solidifying the magnesiumcontainingmelt in a mold.

The invention is particularly useful in the manufacture of ingots whichare cast from the melt in a chilled metal mold. Such ingots uponremelting under vacuum in a refractory crucible have greatly'reducedtendency to wet such crucible, as compared with ingots of the same alloywhich have been similarly prepared but without the magnesium treatment.

The magnesium treatment in accordance with the invention involves theaddition to the melt of enough magnesium so that the metal after castingand cooling to solidified form in vacuum retains magnesium metal in thespecified concentration range (0.00l to 0.02 per cent). The magnesiumpreferably is incorporated in the melt within a relatively short periodprior to casting, to insure against excessive loss of the magnesium byvolatilization. Preferably, the molten alloy is cast into the mold, andcooling of the metal therein is commenced, within thirty minutes afteraddition to the melt of the magnesium.

The magnesium is preferably introduced into the melt in the form of anickel-magnesium master alloy, such as commercially available masteralloys containing 15% by weight magnesium, balance commercially purenickel, but it may be introduced in any other suitable manner.

It is a significant advantage of the invention that ingots and the likewhich show little tendency to wet refractory melting crucibles can beprepared from charges containing a large proportion of foundry revert aswell as from charges consisting of virgin metal.

SPECIFIC EMBODIMENTS OF THE INVENTION Alloys having the followingcompositions are prepared by melting under vacuum appropriate quantitiesof the metal involved '(all percentages are by weight of the alloy):

The alloys may be prepared in whole from virgin metal, or from majoramounts of revert to which amounts of virgin metal are added as requiredfor composition adjustment and to make up the desired weight of alloy.The alloys are prepared in accordance with conventional practice bymelting revert and virgin metal additions under vacuum in an inductionmelting furnace. The virgin metal may be added to the charge either ascommercially pure metal or as master alloy (generally a nickel masteralloy) in conformity with commercial practice.

The melt is prepared as essentially oxygen-free and sulfur-free metal,by use of charge components that are themselves essentially free ofthese impurities. To the extent that very small proportions of oxygen orsulfur may be included in the charge, they are eliminated by thedeoxidizing and desulfurizing effect of such constituents as zirconiumand boron.

When the melt has been brought to the proper oxygen-free and sulfur-freecomposition and to the desired temperature, magnesium in the form ofnickel magnesium master alloy percent by weight magnesium) is added inan amount sufficient to insure retention of 0.001 to 0.02 percentmagnesium in the alloy after casting and solidification under vacuum.Generally this requires the addition of 0.05 to 0.10 percent by weightofmagnesium to the melt, to allow for losses by volatilization.

Shortly after addition of the magnesium and dispersal of it through themelt, the melt is cast in vacuum into a suitable mold such as a chilledmetallic ingot mold. Casting preferably is within thirty minutes afteraddition of the magnesium, and in a typical case will be within fifteento thirty minutes thereafter. However, the time between adding themagnesium and casting may exceed thirty minutes. Castings of goodquality and low tendency to wet refractory crucibles upon remelting havebeen poured over two hours after making the magnesium addition.

As would be expected, the magnesium content of the castings varieswiththe length of time between magnesium additions and pouring of the ,meltinto the mold.

' First-poured ingots from a given melt generally have substantiallymore retained magnesium (typically 0.008 to 0.013 percent) thanlast-poured ingots from the same melt (which typically may contain 0.001to 0.002 percent). However, there is no significant correlation betweenthe amount of retained magnesium in ingots cast from a given melt, andthe non-wetting character of such ingotsrelative to refractory surfaces.It appears that the magnesium treatment of the melt prior to casting ofthe ingots, rather than the actual amount of retained magnesium, isresponsible for the tendency of the metal not to wet refractorysurfaces.

The effectiveness of the magnesium treatment may be demonstrated, ortested, without actually remelting a cast ingot in a refractorycrucible, by dipping a refractory test bar or plaque into the melt fromwhich the ingots are cast and observing to what extent the molten metaladheres when the test bar or plaque is withdrawn. Nickel-base alloymelts, after being subjected to the magnesium treatment described,generally display notably less tendency to wet such refractory testpieces than do the same melts prior to the magnesium treatment.

lngots of nickel-base superalloys prepared as described above may beremelted in vacuum in refractory crucibles, without further magnesiumtreatment, without significantly wetting such crucibles. Hence there islittle tendency for melts prepared from such ingots to becomecontaminated with loose refractory particles from the crucible surfaceeven after repeated cycles of ingot addition, melting and cooling in thesame crucible. The quality of castings made from such ingot melts iscorrespondingly improved. This improvement is especially notable in thecase of ingots made from revert metal.

We claim:

1. In the vacuum casting of nickel-base superalloys consistingessentially (by weight of the alloy) of 7 to 20 percent chromium, 0.5 to7 percent aluminum, 0.5 to 6 percent titanium, 0.02 to 0.25 percentcarbon, up to 0.2 percent zirconium, up to 0.02 percent boron, up to 3percent hafnium, and at least 10 percent of an amount of amatrix-strengthening element selected from the group consisting of 0 to20 percent cobalt, 0 to 10 percent molybdenum, 0-13 percent tungsten,0-6 percent tantalum, 0-5 percent columbium and 0 to 1.5 percentvanadium, and the balance nickel, the method of minimizing tendency ofthe alloy in the molten state to wet and upon solidification to adhereto a refractory vessel in which it is contained under vacuum in themolten condition which comprises introducing into a substantiallyoxygen-free and sulfur-free melt of such alloy an amount of magnesiumsufficient to incorporate in cooled and solidified shapes cast therefroma residual retained amount of elementalmagnesium in the range from,0.001 to 0.02 percent by weight of the al- 10y, and casting andsolidifying the magnesiumcontaining melt in a mold.

2. The method according to claim 1 in which the molten metal is castinto the shape of ingots for remelting.

a nickel-magnesium master alloy.

1. IN THE VACUUM CASTING OF NICKEL-BASE SUPERALLOYS CONSISTINGESSENTIALLY (BY WEIGHT OF THE ALLOY) OF 7 TO 20 PERCENT CHROMIUM, 0.5 TO7 PERCENT ALUMINUM, 0.5 TO 6 PERCENT TITANIUM, 0.02 TO 0.25 PERCENTCARBON, UP TO 0.2 PERCENT ZIRCONIUM, UP TO 0.02 PERCENT BORON, UP TO 3PERCENT HAFNIUM, AND AT LEAST 10 PERCENT OF AN AMOUNT OF AMATRIX-STRENGTHENING ELEMENT SELECTED FROM THE GROUP CONSISTING OF 0 TO20 PERCENT COBALT, 0 TO 10 PERCENT MOLYBDENUM, 0-13 PERCENT TUNGSTEN,0-6 PERCENT TANTALUM, 0-5 PERCENT COLUMBIUM AND 0 TO 1.5 PERCENTVANADIUM, AND THE BALANCE NICKEL, THE METHOD OF MINIMIZING TENDENCY OFTHE ALLOY IN THE MOLTEN STATE TO WET AND UPON SOLIDIFICATION TO ADHERETO A REFRACTORY VESSEL IN WHICH IT IS CONTAINED UNDER VACUUM IN THEMOLTEN CONDITION WHICH COMPRISES INTRODUCING INTO A SUBSTANTIALLYOXYGEN-FREE AND SULFUR-FREE MELT OF SUCH ALLOY IN AN AMOUNT OF MAGNESIUMSUFFICIENT TO INCORPORATE IN COOLED AND SOLIDIFIED SHAPES CAST THEREFROMA RESIDUAL RETAINED AMOUNT OF ELEMENTAL MAGNESIUM IN THE RANGE FROM0.001 T 0.02 PERCENT BY WEIGHT OF THE ALLOY, AND CASTING AND SOLIDIFYINGTHE MAGNESIUM-CONTAINING MELT IN A MOLD.
 2. The method according toclaim 1 in which the molten metal is cast into the shape of ingots forremelting.
 3. The method according to claim 2 in which the molten metalis cast into a chilled metal mold.
 4. The method according to claim 1 inwhich the molten alloy is cast into a mold and cooling of the metal iscommenced within thirty minutes after addition thereto of the magnesium.5. The method according to claim 1 in which the superalloy melt isprepared at least in part from revert metal from previously madecastings.
 6. The method according to claim 1 in which the magnesium isintroduced into the alloy in the form of a nickel-magnesium masteralloy.